In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. One widely used method of accomplishing this is to process the wood fibers in a cooking liquor so that the material holding the fibers together, lignin, is dissolved. In order to achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper which reduces the raw wood to chips.
As a natural consequence of the harvesting and processing of pulp logs, some rocks and tramp metal find their way into the raw wood chips. Further, a certain percentage of the raw wood is comprised of knots which are in general undesired in the papermaking process because they add dark fibers which increase the bleaching requirement and because they contain resinous material. The knots, which are typically of a higher density because the wood is dense and resinous, together with tramp metal and rocks, must be separated from the raw wood chips before further processing.
One highly successful method of accomplishing this separation is the air density separator. In one known successful system, chips are supplied by a metering screw conveyor infeed to a separation chamber through which a stream of air is drawn. The chips are entrained in the air stream while the higher density knots, stones and tramp metal move against the current of air under the force of gravity. The acceptable chips and air then pass into a cyclone where the chips are separated from the air, the air being drawn by a vacuum into a fan and exhausted.
While the air density separator is the most effective and discriminating system available, it has some less desirable features. First, it requires an air lock for the removal of the accepted chips from the cyclone. The discharge air lock is expensive to maintain and prone to plugging on long stringy materials. Second, while the known air density separators can lower costs because of the complete freedom of the placement of the process equipment, the equipment can nevertheless occupy a substantial area within the chip processing plant. Third, the air density separator consumes considerable energy.
What is needed is an air density separator which functions without an air lock, is more compact in design, and is more energy efficient.